In order to improve the lubricity and the wear resistance of thermoplastic resins such as nylon and polyacetal which have been used as bearing materials, etc., it has been the practice to incorporate into these resins liquid or solid lubricants such as molybdenum disulfide, graphite and polytetrafluoroethylene.
However, molybdenum disulfide, polytetrafluoroethylene and the like are expensive and do not achieve the desired results to a high degree. Moreover, their presence adversely affects the moldability of the resin. A particular problem is that when nylon, polyacetal or similar thermoplastic resin with an added lubricant is subjected to a molding process such as by extrusion molding or screw injection molding technique, the blend fails to fill the molding cavity completely and conform faithfully to its contours. Consequently, the desired molded products are not obtained. To avoid the difficulty, compression molding or the method whereby a sintered resin is immersed in a liquid lubricant are normally employed. With these methods, however, it is difficult to efficiently produce large numbers of molded products of complicated shapes.
Recently a method has been disclosed which enables an oil to be incorporated in nylon or polyacetal (Japanese Official Gazette of Patent Publication No. 5321/1971). This method requires the resin to be treated at an elevated temperature for an extended period of time in the lubricant oil. Unfortunately, this gives rise to the possiblity of degradation of the resin. In addition, this method requires special mechanical equipment which increases the cost and decreases the productivity. Moreover, pellets produced by this method, when subjected to extrusion molding or screw injection molding, tend to bleed out the lubricant which seriously impairs the efficiency of the molding machine. This method, despite elaborate precautions produces molded products in which the surface is wet with lubricant.
It has now been discovered that the addition of a third component, namely an ethylene-vinyl acetate copolymer substantially alleviates the problems aforesaid, and permits the production of molded products of complicated design which have excellent lubricity without a wet surface, and excel in wear resistance. Moreover, the products can be produced at high production rates.